The U.S. Department of Transportation (DOT) and the U.S. Environmental Protection Agency (EPA) have established U.S. federal rules that set national greenhouse gas emission standards. Beginning with 2012 model year vehicles, automobile manufacturers required that fleet-wide greenhouse gas emissions be reduced by approximately five percent every year. Included in the requirements, for example, the new standards decreed that new passenger cars, light-duty trucks, and medium-duty passenger vehicles had to have an estimated combined average emissions level no greater than 250 grams of carbon dioxide (CO2) per mile in vehicle model year 2016.
Catalytic converters are used in internal combustion engines to reduce noxious exhaust emissions arising when fuel is burned as part of the combustion cycle. Significant among such emissions are carbon monoxide and nitric oxide. These gases are dangerous to health but can be converted to less noxious gases by oxidation respectively to carbon dioxide and nitrogen/oxygen. Other noxious gaseous emission products, including unburned hydrocarbons, can also be converted either by oxidation or reduction to less noxious forms. The conversion processes can be effected or accelerated if they are performed at high temperature and in the presence of a suitable catalyst being matched to the particular noxious emission gas that is to be processed and converted to a benign gaseous form. For example, typical catalysts for the conversion of carbon monoxide to carbon dioxide are finely divided platinum and palladium, while a typical catalyst for the conversion of nitric oxide to nitrogen and oxygen is finely divided rhodium.
Catalytic converters have low efficiency when cold, i.e. the running temperature from ambient air start-up temperature to a temperature of the order of 300 C or “light-off” temperature, being the temperature where the metal catalyst starts to accelerate the pollutant conversion processes previously described. Below light-off temperature, little to no catalytic action takes place. This is therefore the period during a vehicle's daily use during which most of the vehicle's polluting emissions are produced.